By Conor Doolin
Chromosomes are packages of genetic material. Females get two X chromosomes and males get both an X and a Y chromosome. Sex determination is quite variable. Scientists hypothesize Xs and Ys evolved independently in several species. Over the last decade, there has been concern that the Y chromosome may evolve itself into extinction! Single mammalian eggs carry one large sex chromosome X, and a single mammalian sperm carry either one large sex chromosome (X), or one SMALL sex chromosome (Y). Egg and sperm cells are collectively called gametes. Typically, offspring contain ½ copies of a parent’s genome and are all unique. When two gametes merge, the offspring genome differs from both its parents’ and its siblings’. One reason for these differences is resilience, where genetic diversity leads to individuals with different traits. As environments change, variations help future generations of organisms adapt. This ensures that at least some individuals in the species will survive long term. Scientists hypothesize, “climate and ecological factors cause sex-specific demographic changes” [2].
The whole system of sex determination is an evolving biological mechanism. The SRY gene controls the development of male sex traits. There are hypotheses that sex-determining genes occurred through mutation(s). Viable chromosomes undergo a process known as recombination, in which adjacent chromosomes swap DNA. Y chromosomes, though, just stopped recombining for some reason. This leads chromosomes to lose bits and pieces and erode over time. In fact, evidence suggests that Y chromosomes can degrade so much they go extinct in whole species, this is known as, “the fragile Y hypothesis” [1]. A modern example is the male species of Amami spiny rat. Amami spiny rats give live birth, but the males have no Y chromosome, which has slowly been shed over half a billion years of evolution [2]. This Y chromosome loss appears inescapable. Researchers noticed that Y chromosomes once carried 1,400 genes (like X chromosomes), but have been shedding traits over the last 200 million years to the point where only 27 are left, including traits controlled by the SRY gene. Scientists think Y degradation began as a protective measure. If Y chromosomes over exchanged SRY genes then all chromosomes would encode male traits. Therefore, the Y chromosome evolved a “safety mechanism”, no genes get in and no genes get out. Essentially, an abundance of Y genes becomes useless if there are not ample X genes to continue reproducing and diversifying species.
Research suggests complete Y chromosome extinction is possible in the next 5 million years though unlikely. The Y chromosomes are probably not going anywhere. Ultimately, “the saving grace for the Y could be its selfish nature to recruit and retain genes from across the genome that have male beneficial function” [1]. This biological adaption proves natural selection is still at work in the Y chromosome. Maybe the Y chromosome was not shrinking, but it was just becoming more efficient!
References
[1] P. D. Waters and A. Ruiz-Herrera, “Meiotic Executioner Genes Protect the Y from Extinction,” Trends Genet., vol. 36, no. 10, pp. 728–738, Oct. 2020, doi: 10.1016/j.tig.2020.06.008.
[2] T. C. Zeng, A. J. Aw, and M. W. Feldman, “Cultural hitchhiking and competition between patrilineal kin groups explain the post-Neolithic Y-chromosome bottleneck,” Nat. Commun., vol. 9, no. 1, Art. no. 1, May 2018, doi: 10.1038/s41467-018-04375-6.